Why does it say "Ach Ish Al Yarev [v'Al Yochach Ish]"?
Rashi: You warn the true Nevi'im not to quarrel and rebuke you. Amatzyah Kohen Beis Kel told Amos "Lo Sinavei Al Yisrael v'Lo Satif Al Beis Yischak" (Amos 7:16). He was in the days of Yaravam ben Yo'ash; Hoshe'a prophesized in his days.
Radak: Here, "Ach" does not exclude. Rather, it teaches the truth of the matter, like "Ach Atzmi u'Vesari Atah" (Bereishis 29:14), "Ach la'Sheker Shamarti" (Shmuel I, 25:21).
What do we learn from "v'Amecha ki'Merivei Chohen"?
Kidushin 70b: Brazenness is normal among (even proper) Kohanim.
Rashi citing Targum Yonasan: They quarrel with their teachers. Kohanim were teachers ? "Yoru Mishpatecha l'Yakov" (Devarim 33:10). One should not quarrel with his friend or rebuke him for his evil, for it will not help ? also he does evil, like him!
Radak #1: The prefix Kaf in ki'Merivei teaches the truth of the matter, like "Hishav'ah Li ka'Yom" (Bereishis 25:33), 'k'Masigei Gevul" (below, 5:10). The Kohen should have taught, quarreled with and rebuked the nation - "Yoru Mishpatecha l'Yakov", "u'Vasa El ha'Kohen" (Devarim 33:10, 26:3). Nowadays, not only do they not accept his rebuke ? they even quarrel with and rebuke him, like 'a generation that judged its judges' (Bava Basra 15b). Malbim ? they tell the Kohen "v'Chashalta ha'Yom v'Chashal Gam Navi Imecha" (5).
Radak #2: The Kohen is evil like them. If he will rebuke them, also they will rebuke him.
Radak: Some say that this is like Korach's congregation, who contested and quarreled about the Kehunah.
Malbim: Refer to 4:5:2:3.
